Docs: refactor Upgrades for optional features section

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Problem to solve

Upgrades for optional features section mentions Geo, external Gitaly, KAS, ES, however there are other options how customers can configure environment and would need to consider for upgrades. For example, external PG, Redis or general multi-node setup. Technically Geo or separate Gitaly instance is not an optional feature, but how customer built their environment.

Proposal

Group upgrade considerations by theme - please see !208253 (comment 2813229301)

1. Multi-node GitLab setup / non single node Omnibus - need to follow specific upgrade order of the components, mentioned in with downtime or in ZDU docs
2. Optional component dependencies used? upgrade dependencies to match that version GitLab requirements

Who can address the issue

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